Originally Posted by Pleiades

Stage 3 airbox cover does make a difference, but only with fuelling adjusted to suit. There is a considerable increase in airflow over a Stage 2 setup. Almost all the power gains from fitting a Stage 3 cover will come at the top end from 5000rpm and upwards.

As for the plugs... TBH, I can't tell the difference between a standard and iridium plug on the Z and neither between heat ranges 7 and 8! However, there are plenty of people who swear they can "feel" the difference...